<h2>DESCRIPTION</h2>

<em>i.emissivity</em> calculates the emissivity in the longwave radiation
spectrum, according to the semi-empirical equation related to NDVI by
Caselles et al. (1997), valid in the NDVI range of 0.16 to 0.74 (Bastiaanssen, 1995).
<p>
Caselles et al. (1997) give reference (in Table 3 and Figure 2) to both
the NDVI range used (0.15 - 0.71) and the corresponding emissivity range
used (0.97 - 0.99). 
<p>
The emissivity is the efficiency of longwave energy returning to the
atmosphere from the skin surface. The skin surface receives heat from
the thermal infrared radiation of the Sun, through atmospheric interaction.
A part is returned to the atmosphere fastly, and another part is kept in
the surface skin to be returned later. In more scientific terms, the
grey body radiation is equal to the black body radiation times the emissivity.

<h2>SEE ALSO</h2>

<em>
<a href="i.eb.netrad.html">i.eb.netrad</a>
</em>

<h2>REFERENCES</h2>

<ul>
<li> Bastiaanssen, W.G.M., 1995.
  Estimation of Land surface parameters by remote sensing under clear-sky
  conditions. PhD thesis, Wageningen University, Wageningen, The Netherlands.
  (<a href="http://edepot.wur.nl/206553">PDF</a>)</li>
<li> Caselles, V., C. Coll, and E. Valor, 1997.
  Land surface emissivity and temperature determination in the whole
  HAPEX-Sahel area from AVHRR data. International Journal of Remote
  Sensing 18(5):1009-1027.</li>
<li> Rubio, E., V. Caselles, and C. Badenas, 1997.
  Emissivity measurements of several soils and vegetation types in the
  8-14 &micro;m wave band: Analysis of two field methods. Remote Sensing of
  Environment 59(3): 490-521.
</ul>

<h2>AUTHOR</h2>

Yann Chemin, GRASS Development Team<br>

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